REGULATION OF C-ERBA-ALPHA MESSENGER-RNA SPECIES IN TADPOLE ERYTHROCYTES BY THYROID-HORMONE

Putative thyroid hormone (TH) nuclear receptors have been detected in several tissues of Rana catesbeiana tadpoles. T3 receptor number (sites per nucleus) in red blood cells (RBCs) and tail increases substantially just before metamorphic climax or in response to exogenous TH; in contrast, receptor number in liver remains relatively constant. TH receptors in mammals and birds are thought to be encoded by a c-erbA gene. In the present study, two c-erbA cDNAs, one prepared from Xenopus laevis oocytes (XenTR-alpha-1) and one prepared from Rana catesbeiana tail (RC12), were used to examine the c-erbA-related mRNA species in Rana catesbeiana tissues and determine their role in the TH induction of tadpole RBC receptor number. XenTR-alpha-1 encodes a protein with T3-binding properties typical of TH receptors. RC12 is almost 99% homologous with XenTR-alpha-1 at the amino acid level and contains all of the putative T3-binding region and most of the DNA-binding region. Using either cDNA as a probe, it was found that two major species of c-erbA-related mRNA species (2.6 and 4.0 kilobases) were clearly evident in tadpole RBCs, tail, and liver. A third, more diffuse band (approximately 5.0 kilobases) was observed in RBC and tail. In RBCs, but not in liver, the combined level of c-erbA-related mRNA species was increased during spontaneous metamorphosis or after administration of TH. Furthermore, the TH-induced increase in both c-erbA-related mRNA species and receptor number in RBCs was prevented if actinomycin-D was administered with TH. In view of the high homology between RC12 and XenTR-alpha-1 and the fact that XenTR-alpha-1 encodes a putative T3 receptor, the present data are consistent with the concept that the TH receptor in Rana catesbeiana is also encoded by a c-erbA gene. The data also suggest that TH stimulates receptor number at least in part by enhancing the rate of transcription of this gene.